Gold has long been used in the electronics industry as a material for contact surfaces because of its low electrical resistivity and its inertness to attack by corrosive substances. Contact and connector surfaces of most electronic devices are usually electroplated with gold whenever reliability of the device is required. As the trend toward miniaturization of electronic devices and components continues, there is an increasing need within the industry for small, relatively thick, isolated gold deposits on printed wiring boards and integrated circuit chips. Typical thicknesses required for these deposits are in the range of 0.5 to 2 microns (20 to 80 microinches).
Applications of these deposits include pads for gold wire bonding as well as direct interconnection from one device to another. The fact that the areas where gold deposition is required are isolated from each other prevents electrolytic deposition of gold from being used to produce these deposits since the areas are not electrically connected to one another. Consequently, such deposits can be produced only by electroless deposition of gold.
Electroless deposition is a process by which a metal is deposited onto a surface by autocatalytic chemical reduction. Both a dissolved salt of the metal to be deposited, often in the form of a complex ion, and an appropriate reducing agent are dissolved in the plating solution. In order for such a solution to be of practical use, the reaction between dissolved metal species and the reducing agent must occur only on the surface areas where metal deposition is desired and not in the bulk of the solution. In order for thick deposits to be produced, the reaction between the dissolved metal species and the reducing agent must continue to occur on the surface of the deposited metal. Hence, electroless deposition is often referred to as autocatalytic deposition since the deposited metal acts as a catalytic surface for further deposition of the metal. A generally accepted indication that an electroless deposition process is truly autocatalytic is that its rate of deposition is more or less independent of thickness. That is, thickness of the deposited metal is linear with respect to deposition time at a given temperature.
Other aspects of an electroless plating solution which are of practical consequence--especially when producing electronic devices--are its operating pH and temperature. These must be compatible with other materials on the device such as photoresist when the device is immersed in the solution. If such materials are attacked by the electroless plating solution, deposition can occur on unwanted locations on the device. Consequently, it is most desirable that electroless solutions operate close to neutral pH and at a temperature as low as possible consistent with an adequate deposition rate.
There have been numerous attempts to produce workable electroless gold plating solutions. Many of these are described in reviews by Okinaka (Chapter 15 of "Electroless Plating: Fundamentals and Applications," edited by G. O. Mallory and J. B. Hajdu, American Electroplaters and Surface Finishers Society, 1990, and Chapter 2 of "Advances in Electrochemical Science and Engineering," volume 3, edited by H. Gerischer and C. W. Tobias, VCH Publishers, 1994).
A solution that has received wide attention is that described by Okinaka in U.S. Pat. No. 3,700,469. This solution contained a soluble gold cyanide complex along with excess cyanide and hydroxide ions and used an alkali metal borohydride or dimethylamine borane as the reducing agent. Its operating temperature was 60.degree. to 95.degree. C. Although much work has been reported on attempts to improve the performance of this formulation, it still suffers from two major problems. First, the reducing agent undergoes a hydrolysis reaction whose rate increases with temperature. Thus, much of the reducing agent is lost in an undesired side reaction making control of its concentration quite difficult if the bath is to be maintained at its operating temperature for a duration of several hours of longer. Second, the solution is subject to spontaneous decomposition whereby the dissolved gold is reduced homogeneously in the solution to produce a metallic gold sludge. No reported modifications of this solution have yet solved these problems in a practical manner. Even if a solution to these problems were found, the fact that the solution operates at a high pH and a high temperature would still render it incompatible with many materials used in present-day electronic packaging technology.
More recent efforts to develop autocatalytic gold deposition solutions that are both stable and compatible with materials that are used in the packaging of electronic devices have focused on the use of non-cyanide gold complexes which can be reduced in solutions of near-neutral pH. Examples of such gold complexes include gold (III) chloride (U.S. Pat. Nos. 5,198,273; 5,202,151; and 5,470,381), gold (I) thiosulfate (U.S. Pat. Nos. 4,804,559; 5,198,273; 5,202,151; 5,318,621; and 5,470,381), and gold (I) sulfite (U.S. Pat. Nos. 5,318,621; 5,364,460; and 5,470,381). Suitable reducing agents for these gold complexes include thiourea and its derivatives (U.S. Pat. Nos. 4,804,559; 4,880,464; 5,198,273; and 5,202,151), amine boranes or hydroquinone (U.S. Pat. No. 5,364,460), amino acids (U.S. Pat. No. 5,318,621), and ascorbic acid or a salt thereof (U.S. Pat. Nos. 5,364,460 and 5,470,381).
It has generally been found that the solutions described above are lacking in one or more features that would be desired of a solution for use in commercial production applications. For example, the solutions which use thiourea as a reducing agent must be heated as hot as 80.degree. to 90.degree. C. in order to achieve acceptable deposition rates. Such temperatures are too high for use with some electronics packaging materials. Also, at these temperatures the solutions can easily become unstable and spontaneously form fine particles of gold throughout the solution instead of producing gold deposits only on the desired substrate. When this occurs, the solution is said to "plate out" or "crash" and any further use of the solution is precluded. A common approach to eliminate or lessen the amount of plate-out that occurs in a solution is to add a stabilizer to the solution. The use of various stabilizer compounds in autocatalytic gold solutions is described, for example, in U.S. Pat. Nos. 5,364,460 and 5,470,381.
Another problem associated with the aforementioned electroless gold solutions is the difficulty in operating these solutions for extended periods of time while replenishing the ingredients that are depleted during deposition such as the gold complex, the reducing agent, and the stabilizer or stabilizers. If these ingredients are not maintained near their optimum concentrations, the solution will not operate satisfactorily. Either the deposition rate will decrease to an unacceptably slow rate if the gold complex or the reducing agent become depleted or the solution will spontaneously decompose if their concentrations are allowed to increase beyond certain limits. Likewise, if the stabilizer concentration becomes too high, the deposition rate of gold will diminish and if the stabilizer concentration becomes too low, the solution can spontaneously plate out. In many instances, it is difficult to control such solutions so that they can be used for several metal turnovers. A metal turnover is defined as the deposition of an amount of metal equal to that initially dissolved in the solution.